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View Full Version : Can someone explain the ball diff


Spoon
2006.02.15, 10:49 AM
I was having some handling problems (understeer) on my MR02 last night so I searched the forum and found a couple of results. The one that I decided to try was loosening my ball diff just a little bit and it seemed to help.

I usually try to set my diff so if I grab the wheels and pull the throttle, the spur moves just a little bit. I noticed it was a little tighter than that so I loosened it up so that the spur moves more than just a little and the understeer went away. I think I did lose a little bit of acceleration though.

Could someone explain what happens to the power at the wheels when I loosen the ball diff? I assumed that I would have to try something up front (different springs, different toe) to get rid of the understeer.

For reference, here is my setup:

MR02
94mm Mid Mount
Stock knuckles
stainless kingpins
yellow (hard?) springs
Kyosho oil damper (stock oil)
Kyosho Ball Diff
Soft H-plate

byebye
2006.02.15, 11:26 AM
I would clean the diff and light lube it. Tighten it up more than normal. Try a medium h-plate. Make sure that the rear end has equal play from side to side. Make sure that the front is traveling normal. Rotate your front wheels, tires and bearings and make sure the shaft of the knuckle is clean. Check your pinion. Check the play in the ball diff from side to side(there should be a tiny bit for good measure). Check for rubbing.

-Byebye

Spoon
2006.02.15, 11:36 AM
I think I have worked out the problem, but I was hoping someone could explain to me what happens on a turn with a tight diff and a loose diff. For example on a tight right turn with a loose diff, is there more power on the inside wheel, outside wheel or is it even. If the diff is locked, how does the effect my turn?

Everything is pretty clean and moving freely. I could stand to lube the diff...and unfortunately, I only have one H-plate.

SuperFly
2006.02.15, 12:01 PM
How the diff functions at various states of tightness depends a little bit on how you drive.

For example, if I'm having an understeer problem, I would tend to tighten the diff and use more power in the turn. Because the diff is tighter, the wheels will break traction with the track and spin a little, causing my back end to drift (hate using that word) a little and tighten my turning radius.

To answer your question specifically, if you are in a tight turn with a loose diff: think of your diff kind of like a clutch, where some of the motor's power is dissipated within the bearings, plates and grease. With a loose diff, ideally both tires stay planted, power is distribued evenly, and excess power is dissipated within the diff. You can hear it, by the way. When you hear your diff making a high-pitched whine, that is your spur gear spinning within the two plates.

If you haven't done it before, disassembling a diff (to clean it out and re-grease, maybe polish the plates) can be really helpful in understanding how it functions. Basically you have all your power being transfered from the motor to the spur gear, which has the "balls" fitted within. The friction (or lack thereof) between the the points of contact of the balls and the plates causes the balls to either grip the surface of the plate, roll along the surface, or slide around it. Which of those things it is doing depends on how tight you have it, and how much and what type of grease you have in there.

Spoon
2006.02.15, 12:31 PM
How the diff functions at various states of tightness depends a little bit on how you drive.

For example, if I'm having an understeer problem, I would tend to tighten the diff and use more power in the turn. Because the diff is tighter, the wheels will break traction with the track and spin a little, causing my back end to drift (hate using that word) a little and tighten my turning radius.

To answer your question specifically, if you are in a tight turn with a loose diff: think of your diff kind of like a clutch, where some of the motor's power is dissipated within the bearings, plates and grease. With a loose diff, ideally both tires stay planted, power is distribued evenly, and excess power is dissipated within the diff. You can hear it, by the way. When you hear your diff making a high-pitched whine, that is your spur gear spinning within the two plates.

If you haven't done it before, disassembling a diff (to clean it out and re-grease, maybe polish the plates) can be really helpful in understanding how it functions. Basically you have all your power being transfered from the motor to the spur gear, which has the "balls" fitted within. The friction (or lack thereof) between the the points of contact of the balls and the plates causes the balls to either grip the surface of the plate, roll along the surface, or slide around it. Which of those things it is doing depends on how tight you have it, and how much and what type of grease you have in there.


That's starting to make sense. With the diff looser am I basically scrubbing some speed/power giving the front wheels more ability to hold traction?

I am sure there are threads on cleaning and lubing the diff. What kind of grease do you use? Any suggestions for cleaning/polishing the diff?

SuperFly
2006.02.15, 01:01 PM
That's starting to make sense. With the diff looser am I basically scrubbing some speed/power giving the front wheels more ability to hold traction?

I am sure there are threads on cleaning and lubing the diff. What kind of grease do you use? Any suggestions for cleaning/polishing the diff?

Yeah, that's basically it. There's some other things going on as well, like allowing each of the rear wheels to rotate at a different rate. You can picture what will happen to the inner wheel with a solid axle (or locked diff); the outer wheel has to travel quite a bit more than the inner wheel, so without diff slippage, either the inner wheel will spin on the surface (which is what will happen), or the outer wheel will have to skid.

I just got some PN diff grease from Reflex, and it seems pretty good. As far as polishing the plates, they are just metal washers. I just put sandpaper on a hard, flat surface and then rub the plate in a circular motion. I use 400 grit sandpaper, followed by 800 grit, then a final polish with Brasso metal polish. I then take a toothpick and stick a little blob of grease on each ball and close it back up.

PTRacer
2006.02.15, 02:47 PM
You shouldn't polish the diff rings. These are a friction style differential's, and the ball's need to 'grip' the ring's, otherwise you will lose power and need to tighten the dif too much for it not to slip...

I just sand the 'shine' out of the rings, and with used rings make shure that the small line created by the diff ball's disapears...This way you can use your dif rings forever :D

SuperFly
2006.02.15, 04:00 PM
You shouldn't polish the diff rings. These are a friction style differential's, and the ball's need to 'grip' the ring's, otherwise you will lose power and need to tighten the dif too much for it not to slip...

I just sand the 'shine' out of the rings, and with used rings make shure that the small line created by the diff ball's disapears...This way you can use your dif rings forever :D


Hmm, yeah, I could see that. My polished ones work good, but I do have them cranked down pretty tight. Either way, the trick is when you're sanding them, don't put too much pressure on it so the scratches the sandpaper makes aren't too deep.

Spoon
2006.02.15, 04:57 PM
All good tips, I'll pick up some diff lube with my next order.